This invention relates to a line connection reversing circuit utilizing semiconductor switches and more particularly to a line connection reversing circuit utilized to form a switching matrix installed in a telephone exchange processing audio, image signals or data.
To have better understanding of this invention a typical line connection reversing circuit or a line switching device utilized in a telephone exchange will first be described.
In this circuit, paired transmission circuits 1, 2 and 3 and another set of paired transmission circuits 4, 5 and 6 are arranged to cross each other and semiconductor switching elements are disposed at respective cross-points. Voltage sources 8, 9 and 10 are connected to paired transmission lines 1, 2 and 3 respectively and impedances 11, 12 and 13 are connected across paired transmission lines 4, 5 and 6, respectively. Conductors of respective paired transmission lines are designated by digits with suffixes a and b, for example 1a, 1b, 2a, 2b and so on. Any cross-point between two paired transmission lines as illustrated by a circle 7 is hereinafter termed a cross-point. Further, an arrangement of four switches or switching elements as illustrated by circle 14 is termed a line reversing switch. With this connection, in order to interconnect any one of paired transmission lines 1 to 3 and any one of paired transmission lines 4 to 6, it is necessary to close the switching elements at their cross-point and the corresponding reversing switch. Thus, to connect paired transmission lines 3 and 4 the switching elements at their cross-point 7 are closed and the switches 15a and 15b of the line reversing switch 14 are closed whereas switches 16a and 16b are opened. Then, the current from the voltage source 10 flows to the impedance 11 through the line reversing switch 14 and the switching elements at the cross-point 7. At this time, current flows through impedance 11 in the direction of arrow 17. Under these conditions when switches 15a and 15b of the line reversing switch 14 are opened and switches 16a and 16b are closed, current flows through the impedance in the direction of arrow 18. In this manner, in order to reverse the direction of current flowing through the impedance without reversing the polarity of the voltage source it is necessary to provide the line reversing switch 14. Moreover, in order to establish a through line it is necessary to control both switching elements at the cross-point and the line reversing switch. This arrangement not only requires a large number of switching elements but also requires a long time to establish a through line.